JP3835872B2 - Surface acoustic wave resonator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a spurious suppression design method for a surface acoustic wave resonator that is indispensable for the configuration of a surface acoustic wave resonator , particularly a resonator-type surface acoustic wave filter.
[0002]
[Prior art]
In general, a surface acoustic wave device has an interdigital transducer (Interdigital Transducer, hereinafter referred to as “IDT”) for exciting a surface acoustic wave (hereinafter referred to as “SAW”), and by processing the IDT. A SAW device can have various characteristics and functions. Conventionally, a SAW device often refers mainly to a SAW filter, and among these SAW filters, a multi-electrode SAW filter has been mainly used. In recent years, research and development of resonator-type SAW filters in addition to multi-electrode type SAW filters has become active, and the term “SAW filter” does not necessarily mean a multi-electrode type SAW filter.
[0003]
FIG. 2 is a plan view of a conventional general SAW resonator.
This SAW resonator has a piezoelectric substrate 1. On the piezoelectric substrate 1, an input terminal 2, an output terminal 3, a SAW excitation IDT 4 having a plurality of electrode fingers made of an aluminum thin film or the like, and an acoustic for returning the SAW leaked from the left and right of the IDT 4 to the IDT 4 A left reflector 5 _l and a right reflector 5 _r having a plurality of electrode fingers made of a reflective aluminum thin film and the like, and a ground bonding pad 6 are formed. Since this SAW resonator is structurally symmetrical on the input side and the output side, the input terminal 2 may be the output terminal and the output terminal 3 may be the input terminal. Usually, the pitch between the electrode fingers of the IDT 4 adjacent to each other and the electrode fingers of the reflectors 5 _l and 5 _r (that is, the distance between the centers of the two adjacent electrode fingers) is the same as the pitch of the electrode fingers of the IDT 4. Designed to be half wavelength of excitation SAW.
On the other hand, since the bonding pad 6 has the same thickness as the IDT 4, if the film thickness of the IDT 4 is 4000 mm or more, the bonding can be performed as it is without providing a gold bonding pad thereon. Since the reflectors 5 _l and 5 _r are formed at the same time as the IDT 4, the material (Al) and the film thickness of the reflectors 5 _l and 5 _r are the same as those of the IDT 4, but may be unnecessary depending on the application. Similarly, IDT4 has some variations depending on the purpose of use.
[0004]
[Problems to be solved by the invention]
However, the conventional SAW resonator has the following problems, which are difficult to solve.
The characteristics of the SAW resonator differ greatly depending on whether the reflectors 5l and 5r are not provided. That is, in the absence of the reflectors 5l and 5r, the SAW leaked from the left and right of the IDT 4 becomes a loss as it is, and the transmission loss of the SAW resonator increases. On the contrary, when there are reflectors 5l and 5r, most of the SAW energy leaked to the left and right of the IDT 4 is reflected by the reflectors 5l and 5r and returns to the IDT 4, so that the transmission loss is low. Spurious is generated in the characteristics, and the use range of the SAW resonator is narrowed.
3 (a) and 3 (b) are diagrams for explaining a conventional problem, and are characteristic diagrams of a SAW resonator without reflectors 5l and 5r. FIG. 3 (a) is an impedance characteristic diagram. FIG. 4B is an insertion loss characteristic diagram. Each characteristic also shows a schematic diagram of the measurement circuit.
The impedance characteristic of FIG. 3A and the insertion loss characteristic of the through state of FIG. 3B behave the same as the impedance characteristic and insertion loss characteristic of a general LC resonator, but there are no reflectors 5l and 5r. Therefore, the SAW energy leaking to the left and right of the IDT 4 becomes a loss, which increases the transmission loss.
[0005]
4 (a) and 4 (b) are diagrams for explaining the conventional problems and are characteristic diagrams of a SAW resonator having reflectors 5l and 5r. FIG. 4 (a) is an impedance characteristic diagram and FIG. FIG. (B) is an insertion loss characteristic diagram.
7a in the impedance characteristic of FIG. 4A is a spurious impedance characteristic, and 7b in the insertion loss characteristic of FIG. 4B is a spurious appearing in the insertion loss characteristic. Since the reflectors 5l and 5r are provided, the transmission loss in the through state shown in FIG. 4 (b) is greatly improved compared to FIG. 3 (b), but the spurious 7b is higher in the region above the center frequency. appear.
On the other hand, when a resonator-type SAW filter, which is one of SAW devices, is configured using SAW resonators, the presence of spurious 7a and 7b is generally not preferable. That is, if the spurious 7a and 7b exist in the frequency band to be used, ripples and absorption bands are generated in the insertion loss characteristic, which may have a serious adverse effect on the filter characteristic. Conventionally, in order to suppress this, there are methods of increasing the thickness of the IDT 4 and reflectors 5l and 5r of the SAW resonator and changing the material to a heavy metal such as Au. In addition to the adverse effects of the spurs 7a and 7b, the problem of adjusting the center frequency, the problem of transmission loss, and the like are added, so that a technically satisfactory one has not yet been obtained.
An object of the present invention is to solve the problems of the prior art and to provide a low-loss SAW resonator by suppressing the adverse effects of spurious by devising the design of the reflector and IDT.
[0006]
[Means for Solving the Problems]
FIG. 5 is an enlarged view of the SAW resonator IDT and the vicinity of the reflector for explaining the principle of the present invention.
The SAW resonator of the present invention includes an IDT 14 having a plurality of electrode fingers, and a reflector 15 having a plurality of electrode fingers is provided on both sides of the IDT 14. Usually, the pitch d of the IDT 14 and the distance d between the reflectors 15 adjacent to each other and the center of the electrode fingers of the IDT 14 are designed to be equal to the half wavelength (= λ / 2) of the wavelength λ of the excitation SAW. In this case, spurious is generated in a certain frequency band. However, if the value of the distance d is changed, the frequency band in which the spurious is generated also changes accordingly, and the spurious returns to the original frequency band every time the distance d increases or decreases by half wavelength. . As a result of theoretical and experimental confirmation, when the distance d is increased from λ / 2, the spurious shifts to a lower frequency range, and when the increase in the distance d reaches λ / 2, the spurious returns to the original position. . Therefore, it can be said that the value of the distance d is an important factor that determines the frequency band in which spurious is generated. Hereinafter, it is assumed that the following conditions are always satisfied unless otherwise noted.
λ / 2 ≦ d <λ
On the other hand, even if only one side (15) of the left and right reflectors of the SAW resonator is left, spurious is generated in the conventional frequency band. However, the level of the loop in the impedance characteristic is, for example, the conventional spurious loop. It turned out to be half of that. Therefore, it can be said that the spurious generated by the reflection of the left and right reflectors is completely independent.
[0007]
Based on the principle as described above, the SAW resonator according to the present invention is provided on the surface of the piezoelectric substrate, the first reflector provided on the surface of the piezoelectric substrate, and the surface of the piezoelectric substrate. A second reflector and on the surface of the piezoelectric substrate, between the first reflector and the second reflector, the first reflector being a first distance; And an IDT provided at a second distance different from the first distance from the second reflector, and a distance between the IDT and the second adjacent to the first reflector. And a third reflector provided on the surface, and a fourth reflector provided on the surface adjacent to the second reflector and spaced apart from the IDT by the first distance. And .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
(Reference example)
The (1) configuration, (2) operation, and (3) advantages of the SAW resonator as a reference example of the present invention will be described.
(1) Configuration
FIG. 6 is a plan view of a SAW resonator showing a reference example of the present invention.
This SAW resonator has a piezoelectric substrate 11 as in the conventional FIG. 2. On the piezoelectric substrate 11, an input terminal 12, an output terminal 13, an excitation IDT 14 having a plurality of electrode fingers, and a SAW The first reflector 15l on the left side as one side and the second reflector 15r on the right side as the other side, and the ground bonding pad 16 are formed. Has been. Although the input terminal 12 and the output terminal 13 are connected to the IDT 14, the input terminal 12 may be used as an output terminal and the output terminal 13 may be used as an input terminal. The IDT 14, the first reflector 15l on the left side, and the second reflector 15r on the right side are surrounded by the bonding pad 16, and the IDT 14, the reflectors 15l, 15r, and the bonding pad 16 have a film thickness of, for example, It is formed of a thin film such as Al having a thickness of 4000 mm or more.
The configuration of the SWA resonator of the present reference example is basically the same as that of the conventional one, but is different from the conventional one in that the distance dl between the left reflector 15l and IDT 14 and the right reflector 15r and IDT 14 are different. The distance dr is set to a different value. As described above, since the spurious generated by the reflections of the reflectors 15l and 15r is independent from each other, for example, the left distance dl is λ / 2 + λ / 8, and the right distance dr is the same as the conventional distance. Set.
[0011]
(2) Operation For example, when a high frequency signal of several MHz or more is input to the input terminal 12 (or the output terminal 13), a high frequency voltage is applied to the electrode finger of the IDT 14 that is electrically connected to the input terminal 12. When a high frequency voltage is applied, a high frequency voltage is inductively generated at the electrode finger of the IDT 14 that is electrically connected to the adjacent output terminal 13 (or input terminal 12). As a result, a potential difference occurs between the output terminals 13. As a result, the surface of the piezoelectric substrate 11 under the electrode fingers of the IDT 14 is distorted, and a SAW having the same frequency as the input high-frequency signal is excited. The excited SAW propagates to the left and right of the IDT 14 and goes toward the outside of the IDT 14. When the SAW reaches the left and right reflectors 15 _l and 15 _r , reflection occurs here, and the SAW reflected from both sides of the IDT 14 is reflected. It goes toward the center of the IDT 14.
As a result, a SAW standing wave is generated in the region between the reflectors 15 _l and 15 _r , and the coupling between the electrode finger on the input terminal 12 side and the electrode finger on the output terminal 13 side is further strengthened. Therefore, a high frequency signal in a certain band is output. Here, since there are reflected waves from the reflectors 15 _l and 15 _r , spurious generation is inevitable, but there are the following advantages.
[0012]
(3) Advantages For example, when the distances dl and dr are equal, the spurious of the left reflector 15l and the spurious of the right reflector 15r are generated in the same frequency band, so the levels are added to form one large spurious. Become. However, since the left-side distance dl and the right-side distance dr are different in this reference example , the frequency bands generated by the spurious of the left reflector 15l and the spurious of the right reflector 15r are different. As a result, the levels are added. Without becoming two independent spurs. This state is shown in FIGS. 7 (a) and 7 (b).
FIGS. 7A and 7B are characteristic diagrams of the SAW resonator of FIG. 6 , FIG. 7A is an impedance characteristic diagram, and FIG. 7B is an insertion loss characteristic diagram. In FIG. 7A, 17al and 17ar are spurious appearing in the impedance characteristic. Further, 17Bl in FIG 7 (b), 17bR is a spurious appearing in the insertion loss characteristic.
Reflectors 15l as shown in FIG. 6, one large spurious generated conventionally placement 15r is, FIG. 7 (a), (b) as shown in a two small spurious 17al, 17ar, 17bl, dispersed 17br Is done. By adjusting the distances dl and dr as described above, the frequency band generated by the spurs 17al, 17ar, 17bl, 17br can be derived to an unnecessary frequency band, and the spurious 17al, 17ar, 17bl, The 17br level can be kept low.
[0013]
(First embodiment)
The (1) configuration, (2) operation, and (3) advantages of the SAW resonator according to the first embodiment of the present invention will be described.
(1) Configuration
FIG. 1 is a plan view of a SAW resonator showing a first embodiment of the present invention . Elements common to those in FIG. 6 showing a reference example are denoted by common reference numerals.
The SAW resonator of the first embodiment is basically the same as the reference example , but the reference example is further developed to divide the left reflector 15l which is one side of the IDT 14 in FIG. For example, the first and third reflectors 15l1 and 15l2 are divided into two equal parts, and the right reflector 15r which is the other side is divided (for example, two second and second reflectors divided into two equal parts) . 4 reflectors 15r1 , 15r2), and the distance from each part of the IDT 14 is set to different values as follows.
The distance between the left side of the equally divided reflectors 15l1,15l2 the IDT14 and dl1, dl2, the distance between the right bisected reflector 15r1,15r2 the IDT14 and dr1, dr2. For example, the distance dl1 is λ / 2 + (2/8) λ, the distance dl2 is λ / 2 + (3/8) λ, the distance dr1 is λ / 2, and the distance dr2 is λ / 2 + λ / 8. Each is set.
[0014]
(2) Operation The operation principle of the SAW resonator according to the present embodiment is almost the same as the operation principle of the reference example , so the description will be simplified. When a high frequency signal is input to the input terminal 12, the SAW is excited by the IDT 14, the excited SAW propagates to the left and right of the IDT 14, and four reflectors having different distances dl1, dl2, dr1, dr2 to the IDT 14 are provided. Reflected by 15l1, 15l2, 15r1, and 15r2 and returns to IDT14. As a result, a standing wave is generated in the same manner as a SAW resonator having a conventional reflector, and a signal in the passband is strongly output from the output terminal 13. Since there are reflectors 15l1, 15l2, 15r1, and 15r2, there is almost no leakage SAW energy, and the output signal level is the same as the conventional one, but it has the following advantages.
[0015]
(3) Advantages In this embodiment, the SAW energy leaked from both sides of the IDT 14 is reflected by the respective reflectors 15l1, 15l2, 15r1, 15r2 and returns to the IDT 14, but the respective reflectors 15l1, 15l2, 15r1, 15r2 Since the distances dl1, dl2, dr1, dr2 between IDT 14 and IDT 14 are different, four reflected waves are generated. When the leaked SAW energy returns to the IDT 14, the SAW resonator of the present embodiment is the same as the conventional SAW resonator in terms of loss. Since each reflected wave generates one spurious, there are four spurious characteristics in the SAW resonator of this embodiment.
FIGS. 8A and 8B are characteristic diagrams of the SAW resonator of FIG. 1. FIG. 8A is an impedance characteristic diagram, and FIG. 8B is an insertion loss characteristic diagram. 18al1, 18al2, 18ar1, and 18ar2 in FIG. 8A are spurious due to the impedance characteristics of the reflectors 15l1, 15l2, 15r1, and 15r2. Further, 18bl1, 18bl2, 18br1, and 18br2 in FIG. 8B are spurious insertion loss characteristics due to the reflectors 15l1, 15l2, 15r1, and 15r2.
The frequency band of the spurious 18ar1 or 18br1 by the reflector 15r1 is the frequency band of the spurious of the conventional SAW resonator, and the frequency band of the spurious 18ar2, 18al1, 18al2 or 18br2, 18bl1, 18bl2 by the reflectors 15r2, 15l1, 15l2 is: It moves to a lower frequency range than the conventional spurious frequency band.
When the reflectors 15l1, 15l2, 15r1, and 15r2 are divided in this manner, the spurious 18al1, 18al2, 18ar1, 18ar2, 18bl1, 18bl2, 18br1, and 18br2 are also dispersed, and the number thereof is divided into the reflectors 15l1, 15l2, 15r1, and so on. It is the same as the number of 15r2, but the size is reduced accordingly.
[0016]
(Second Embodiment)
The conventional SAW resonator and the SAW resonator of the above-described embodiment can be represented by a lumped constant equivalent circuit diagram as shown in FIG. 9, similarly to the crystal resonator and the ceramic resonator.
This equivalent circuit is composed of an inductor L, a capacitor c1 and a resistor r connected in series, and a capacitor C0 connected in parallel therewith. Such a SAW resonator has a resonance frequency and an anti-resonance frequency, and the theory of constituting a band filter that is a SAW device using the SAW resonator has been known for a long time, but a detailed description thereof will be omitted. FIG. 10 shows a configuration example of a bandpass filter using the SAW resonator of FIG. 6 as a reference example .
FIG. 10 is a configuration diagram of a four-stage ladder filter showing the second embodiment of the present invention.
This four-stage ladder filter is configured by using the SAW resonator of FIG. 6 , and is composed of two series arm resonators 22 and 24 and three parallel arm resonators 21, 23 and 25. ing.
11 (a) and 11 (b) show characteristics of a ladder filter constructed using a conventional SAW resonator and a ladder filter shown in FIG. 10, and FIG. 11 (a) shows a conventional SAW resonator. FIG. 10B is an insertion loss characteristic diagram of the ladder type filter configured using the same, and FIG. 10B is an insertion loss characteristic diagram of the ladder type filter of FIG.
[0017]
Conventionally, when a ladder filter is configured using a SAW resonator, it is necessary to adjust the resonance frequency of the series arm resonator and the anti-resonance frequency of the parallel arm resonator as closely as possible. For example, if such a condition is satisfied, the insertion loss characteristic of the ladder filter becomes a characteristic as shown in FIG. In FIG. 11A, 31 indicates a pass band, and 32 indicates spurious. In the conventional ladder type filter, the frequency band with a low insertion loss is the pass band 31, and the other band is the forbidden band. Further, when a ladder filter is configured using a SAW resonator having a conventional reflector, spurious 32 is also generated. As is clear from FIG. 11A, since the series arm SAW resonator has one large spurious, the ladder filter also has one large spurious 32.
On the other hand, if a ladder filter as shown in FIG. 10 is configured using the SAW resonator of the reference example , the insertion loss characteristic is as shown in FIG. In this ladder type filter, the series arm resonators 22 and 24 have two small spurs. Therefore, the ladder type filter also has two small spurs 41 (here, the two spurs are combined into one name). Will have. By using such a SAW resonator of the reference example , one spurious 32 having a strong adverse effect of the ladder filter constituted by the conventional SAW resonator is dispersed into two spurious 41 having a weak adverse effect, and By adjusting the arrangement of the reflectors 15l and 15r, each spurious can be derived to an unnecessary frequency band.
[0018]
(Third embodiment)
In the third embodiment of the present invention, a ladder filter as shown in FIG. 10 is configured using the SAW resonator shown in FIG. 1 showing the first embodiment.
Figure 12 is an insertion loss characteristic diagram of ladder filter of the third embodiment of the present invention constituted by using the SAW resonator of Fig. 1 showing the first embodiment.
As shown in FIG. 12, since the series arm resonator (corresponding to 22 and 24 in FIG. 10) has four small spurs, the ladder filter of this embodiment also has four small spurious 51 (here, four). Of spurious in one name). The size of each spurious 51 is also ¼ that of a ladder filter using a conventional SAW resonator, and ½ of the ladder filter of the second embodiment using the SAW resonator of the reference example. become. Thus, when the spurious 51 is dispersed, the adverse effect of the spurious is further reduced, and the filter characteristics are further improved.
[0019]
In addition, this invention is not limited to the said reference example and embodiment, A various deformation | transformation is possible.
For example, in the above reference examples and embodiments, the distances dl and dr between the left and right reflectors 15l and 15r and the IDT 14 are set to different values, or the left and right reflectors 15l1, 15l2, 15r1 and 15r2 are divided into two equal parts. The four reflectors 15l1, 15l2, 15r1, and 15r2 obtained are all arranged at different distances dl1, dl2, dr1, and dr2 from the IDT 14, but the way of dividing and arranging these reflectors are different. The method is not limited to the illustrated one, and various modifications are possible. Further, the SAW device configured using these SAW resonators is not limited to the four-stage ladder filter as shown in FIG. 10, and may be a device having another configuration.
[0020]
【The invention's effect】
As described above in detail, according to the present invention, the first and third reflectors and the second and fourth reflectors are provided on the left and right sides of the IDT, and the IDT, the first and fourth reflectors, And the distance between the IDT and the second and third reflectors can be made different, so that one conventional spurious can be dispersed into four small spurs. Therefore, the frequency band in which spurious is generated can be derived to an unnecessary frequency band, and the spurious level can be kept low. Accordingly, it is possible to provide a low-loss SAW resonator while suppressing the adverse effects of spurious.
[Brief description of the drawings]
FIG. 1 is a plan view of a SAW resonator showing a first embodiment of the present invention.
FIG. 2 is a plan view of a conventional SAW resonator.
FIG. 3 is a characteristic diagram of a SAW resonator having no reflector.
FIG. 4 is a characteristic diagram of a SAW resonator having a reflector.
FIG. 5 is an enlarged view of the SAW resonator IDT and the vicinity of the reflector for explaining the principle of the present invention.
FIG. 6 is a plan view of a SAW resonator showing a reference example of the present invention.
FIG. 7 is a characteristic diagram of the SAW resonator of FIG.
8 is a characteristic diagram of the SAW resonator of Fig.
FIG. 9 is a lumped constant equivalent circuit diagram of a SAW resonator.
FIG. 10 is a configuration diagram of a four-stage ladder filter showing a second embodiment of the present invention.
FIG. 11 is a characteristic diagram of the conventional ladder filter and FIG.
12 is an insertion loss characteristic diagram of a ladder filter according to a third embodiment of the present invention configured using the SAW resonator of FIG . 1 ; FIG.
[Explanation of symbols]
11 Piezoelectric substrate 12 Input terminal 13 Output terminal 14 IDT
15 reflectors 15l, 15l1, 15l2 left reflectors 15r, 15r1, 15r2 right reflectors 21 , 23 , 25 parallel arm resonators 22, 24 series arm resonators

Claims (3)

A piezoelectric substrate having a surface;
A first reflector provided on the surface of the piezoelectric substrate;
A second reflector provided on the surface of the piezoelectric substrate;
On the surface of the piezoelectric substrate, between the first reflector and the second reflector, at a first distance from the first reflector, and A transducer provided at a second distance different from the first distance ;
A third reflector provided on the surface adjacent to the first reflector and spaced a distance of the transducer from the two;
A surface acoustic wave resonator comprising: a fourth reflector provided on the surface adjacent to the second reflector and spaced from the transducer by the first distance .   2. The surface acoustic wave resonator according to claim 1, further comprising a grounded bonding pad provided on an outer edge portion of the surface of the piezoelectric substrate.   The first and second distances are respectively λ / 2 2. The surface acoustic wave resonator according to claim 1, wherein the surface acoustic wave resonator is within a range of less than λ (λ is a surface acoustic wave wavelength).

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